Displacement measuring interferometers are well known in the art, and are used to measure changes in a position of a measurement object relative to a reference object based on an optical interference signal. A displacement measuring interferometer may generate an optical interference signal by overlapping and interfering a measurement beam reflected from the measurement object with a reference beam reflected from the reference object.
In many applications, the measurement and reference beams have orthogonal polarizations and different frequencies. The different frequencies can be produced, for example, by laser Zeeman splitting, by acousto-optical modulation, or internal to the laser using birefringent elements or the like. The orthogonal polarizations allow a polarizing beam splitter to direct the measurement and reference beams to the measurement and reference objects, respectively, and thereafter combine the reflected measurement and reference beams to form overlapping exit measurement and reference beams. The overlapping exit beams form an output beam that subsequently passes through a polarizer. The polarizer mixes polarizations of the exit measurement and reference beams to form a mixed beam. Components of the exit measurement and reference beams in the mixed beam interfere with one another so that the intensity of the mixed beam varies with the relative phase of the exit measurement and reference beams.
A detector measures the time-dependent intensity of the mixed beam and generates an electrical interference signal proportional to the intensity. Because the measurement and reference beams have different frequencies, the electrical interference signal includes a “heterodyne” signal having a beat frequency equal to the difference between the frequencies of the exit measurement and reference beams. If the lengths of the measurement and reference paths are changing relative to one another, the measured beat frequency includes a Doppler shift equal to 2 vnp/λ, where v is the relative speed of the measurement and reference objects, λ is the wavelength of the measurement and reference beams, n is the refractive index of the medium through which the light beams travel (e.g., air or vacuum) and p is the number of passes to the reference and measurement objects. Changes in the relative position of the measurement object correspond to changes in the phase of the measured interference signal, with a 2π phase change substantially equal to a distance change L of λ/(np), where L is a round-trip distance change (e.g., the change in distance to and from a stage that includes the measurement object).
FIG. 1 illustrates a conventional interferometer device 100 including a light source 110 and receiver 118. Light source 110 is configured to produce a source beam 112 having two beam components with nominally orthogonal polarizations and different frequencies. For example, a first beam component with a first frequency is nominally linearly polarized in the plane of the figure and a second beam component with a second frequency is nominally linearly polarized in a direction perpendicular to the plane of the figure. Interferometer device 100 also includes a target reflector 116, a reference reflector 128, and a polarization beam splitter 126 having a polarizing beam splitting interface 124. For purposes of this description, polarization beam splitter 126 and reference reflector 128 may also be commonly referred to as interferometer 114.
During operation, light source 110 transmits source beam 112 to polarizing beam splitting interface 124 which separates source beam 112 into a measurement beam 120 and a reference beam 121. Reference beam 121 is reflected by polarizing beam splitter interface 124 and travels along a reference path to reference reflector 128, which then reflects reference beam 121 back to polarizing beam splitting interface 124. Simultaneously, measurement beam 120 is transmitted by polarizing beam splitting interface 124 and travels along a measurement path to target reflector 116. Measurement beam 120 is then reflected back from target reflector 116 to polarizing beam splitting interface 124. Measurement beam 120 and reference beam 121 are then recombined after their respective passes to target and reference reflectors 116, 128 to form mixed output beam 122 having an optical interference signal that is detected by receiver 118. Changes in the relative position of the target reflector 116 may be determined by monitoring changes in the phase of the interference signal at a frequency corresponding to a difference frequency between the measurement and reference beams.
FIG. 2 illustrates a conventional single beam interferometer device 200 including a light source 210 and receiver 218. Similar to light source 110 as illustrated in FIG. 1, light source 210 is configured to produce a source beam 212 having two beam components with different frequencies. Interferometer device 200 also includes quarter-wave plates 230, 231, a target reflector 216, a reference reflector 228, and polarization beam splitter 226 having a polarizing beam splitting interface 224. For purposes of this description, polarization beam splitter 226, quarter-wave plates 230, 231, and reference reflector 228 may also be commonly referred to as interferometer 214.
During operation, light source 210 transmits source beam 212 to polarizing beam splitting interface 224, which separates source beam 212 into a measurement beam 220 and a reference beam 221. Reference beam 221 is reflected by polarizing beam splitting interface 224 and is transmitted along a reference path through quarter-wave plate 230 to a reference reflector 228. Reference reflector 228 then reflects reference beam 221 back through quarter-wave plate 230 to polarizing beam splitting interface 224. Simultaneously, measurement beam 220 is transmitted through polarizing beam splitting interface 224 along a measurement path and through quarter-wave plate 231 to target reflector 216. Measurement beam 220 is then reflected from target reflector 216 back through quarter-wave plate 231 to polarizing beam splitting interface 224. After their respective passes to target and reference reflectors 216, 228, measurement beam 220 and reference beam 221 are recombined to form mixed output beam 222 having an optical interference signal that is detected by receiver 218. Changes in the relative position of the target reflector 216 may be determined by monitoring changes in the phase of the interference signal at a frequency corresponding to a difference frequency between the measurement and reference beams.
Despite the advances that have been made in the field of displacement measuring interferometers, measurement errors and inaccuracies still persist when using conventional apparatus and methods. For example, relative errors may exist due to the fact that each interferometer device within an interferometer system is referenced to a different polarizing beam splitter and reference reflector. FIG. 3 illustrates a conventional interferometer system 301 including a plurality of interferometer devices 300. As illustrated, each interferometer device 300 within interferometer system 301 includes a light source 310, a target reflector 316, and a receiver 318. Furthermore, each interferometer device 300 is referenced to a different interferometer 314 (i.e., interferometer 114 or 214 described above in reference to FIGS. 1 and 2, respectively). As a result, upon detection of movement of a target reflector 316 by a corresponding interferometer system 300, it may be difficult to determine whether the target reflector 316 has moved, or whether the reference reflector (not shown) within the corresponding interferometer 314 has moved.
Furthermore, another disadvantage in having multiple polarizing beam splitters and reference reflectors within an interferometer system 301 is that each interferometer device 300 within the interferometer system 301 may require its own window or port 392 through an environmental chamber 394. As known by one of ordinary skill in the art, each port through an environmental chamber may increase the expense of an interferometer system and may decrease the structural integrity of the environmental chamber.
There is a need to increase the accuracy of an interferometer system and of interferometer-based displacement measuring methods. Specifically, there is a need for methods and systems for an interferometer system configured to measure a plurality of targets using a common beam splitter and a common reference reflector.